One fungus to rule them all

Deep inside Costa Rica’s tropical rainforest, a single fungus determines the course of thousands of lives. As mist rises through the trees and the forest begins to wake, the fungus lies below ground, plotting its strategy to survive another day in one of the most diverse ecosystems in the world.

“We’re gonna have a great day, we’re gonna get things done, we’re gonna rock!” Billy Kinn addresses his team in their office, nestled between moss-covered trees at La Selva Biological Station. Billy and his teammates, Anna DiPaola and Suzy Henderson, are undergraduate students who have flown more than three thousand miles from the United States to study a species that creates one of the most complex animal societies on earth: the leafcutter ant. While the students gather data sheets and zip-lock bags, their subjects are already “rocking,” feverishly excavating a labyrinth of underground chambers and tunnels to construct a nest the size of a full-length basketball court. This dark, damp fortress is where the fungus waits.

One by one, the students file out of the office and disappear into the dense forest. Anna leads the way, and her quick and confident stride assures her teammates that she knows exactly where they are headed. Marching single file alongside her muddy rubber boots are hundreds of leafcutter ants, many of which carry pieces of bright green leaves that are up to 50 times the ants’ body weight. “It’s like a bread crumb trail,” Anna explains. “Sometimes I end up in the middle of the forest without knowing how to get back, and all I need to do is follow them.”

Every single move these ants make, and subsequently each step the researchers will take today, is dictated by the fungus hidden deep within the ants’ nest. The leafcutters are racing against the clock to bring pieces of leaves to this fungus, on which their fate hangs. The fungus breaks down the leaf clippings and grows nutrient-rich tubes that feed millions of ants. In this sense, it serves as an external stomach for the entire colony. Scientists refer to the chamber where this fungus grows as the “fungal garden.”

Suzy bends her narrow frame under twisting vines as the team proceeds deeper and deeper into the forest. ““Vegetarianism is in right now,” she says, giggling. But the leafcutters “have been hippie vegetarian farmers for over 65 million years!”

Inside the fungal garden, the fungus grows around tree roots like cotton candy on a stick. It is crucial to the survival of the colony that the fungus thrives, as it is the ants’ only food source. A group of caretaker ants diligently tend to its every need, and pressure is high for them to provide the best leaf material while monitoring the overall health of their benevolent dictator.

If the fungus is not happy with the type of leaf brought in by its servants- some leaves contain deadly compounds that could slowly kill the fungus over time- it alarms its caretakers by sending out a chemical signal that the ants can interpret, thanks to centuries of genetic mutations and adaptations. It is then their responsibility to relay this information to the ants harvesting these death-sentence leaves far away from the nest. Through a series of smells passed from one ant to the next, the frontline ants receive the message and immediately desert the toxic tree to frantically continue their quest for the perfect leaves. These mistakes are costly, and could very well cause their colony to perish alongside its puppet master.

Meanwhile, five feet above the forest floor, the students stop in their tracks. “We’re here,” Anna says as she surveys her surroundings. At first glance, this patch of forest does not look any different from the terrain they have been navigating. However at closer examination, the earth starts to look like wet sand or tiny balls of clay. One patch is terraced and dotted with several dark holes, resembling ancient pueblo cave dwellings. They have reached the nest. Over fifty of these jelly-bean sized entrances allow ants to haul in leaf clippings and excavate dirt to construct new tunnels. The holes also serve as vents for gas that the fungus releases while it decomposes its salad of the day.

What is visible to the human eye above ground is only a small fraction of the vast nest this society calls home. Leafcutter ants can extend their nest deep into the earth, all the way down to the equivalent of a two-story building. The students, however, are for now only interested in the soil at the surface of the nest. They start scooping it up and placing it in labeled plastic bags. The soil breathes, they explain, in part due to the constant breakdown of leaf matter by the fungus. They want to know how much of the soil’s exhalation is carbon dioxide entering the atmosphere. With this information, they hope to better understand the role leafcutter ants play in greenhouse gas emissions.

As a steady rain begins to fall, the researchers quicken the pace of their soil collection, while the ants exposed to the elements abandon their leaf pieces and scramble to the nearest nest entrance. Below, ants work vigorously to block off tunnels and chambers for fear of flooding. The fungus is extremely sensitive to moisture.

The students retreat as well, and back at the field station, covered in mud and soaked to the core, they trudge into their office. The door is labeled simply “Lab 9,” giving off a quantitative and analytical air. Yet taped below is a beautiful swirl of colors, a detailed drawing illustrating a cross section of the adjacent forest. Lush, green crowded trees blur together and fade back into an endless grove, and below the surface, tangled roots overlap with an elaborate expanse of intricate burrows. Not pictured, but nonetheless safe and dry, decomposing a fresh leaf, the fungus has indeed survived yet another day.

Katie Friedman received her M.S. from Clark University in 2013 exploring salt marsh resilience to anthropogenic nitrate pollution. Currently, she works for the New York State Department of Environmental Conservation where she coordinates citizen science research along the tidal Hudson River Estuary
By: Katie Friedman

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